Many electrical and electronic products require more current input when starting or when increasing load levels than while running with a steady load. Notable among such devices are incandescent lamps and motors. Incandescent lamps have changed life in innumerable ways during the century since their introduction despite their many shortcomings, such as inefficiency, fragility, and filament noise. Also, the tungsten filaments in general use have less resistance when cold than when warm. This causes a cold lamp to draw more current than a warm one (sometimes more than 10 times as much). In high power dimmers, cold lamp filaments may behave little differently from a short circuit.
The lighting industry has dealt with the situation in many ways, such as specifying circuit breakers with a time delay before tripping to allow lamps to warm up, or, in the case of dimmers, applying a small amount of voltage to the lamp to keep the filament warm without giving off a significant amount of light. Incandescent lamp dimmers typically use components with surge current ratings many times their steady state ratings. Thyristors, such as triacs or SCRs (silicon controlled rectifiers), are built with this type of surge current capability. However, they have several drawbacks which have been accepted by the lighting industry as inevitable concomitants: they turn on abruptly, and they cannot be turned off once they start to conduct. The abrupt turn-on causes EMI (electromagnetic interference) and acoustical noise in lamps, connectors and wiring. These problems are reduced by using a series choke to limit the rate of current rise; however, for large currents this choke becomes large, heavy and expensive. The inability to turn off current becomes a problem if an overload or short circuit occurs at the dimmer output. The device must conduct whatever current the power line is capable of delivering until the circuit breaker (or fuse) opens, or until the load current drops to zero at the end of the half-cycle in which the short occurs. High quality dimmers use thyristors rated to handle these very large surge currents; such devices are understandably expensive.
Dimmers which use transistors rather than thyristors and chokes to control current flow to lamp or motor loads manage the current surge and overload situation differently. Transistors do not, in general, have the surge current ratings to handle a cold lamp load of the full nominal dimmer rating, let alone the current of a short circuit. Transistor dimmers rely on current sensing to detect potentially damaging currents, and turn off or limit current to prevent damage.
This is adequate for handling short circuits; all that is usually required is that the dimmer suffer no damage. However, when cold lamp filaments are the cause of a current surge, current limiting causes a slowdown in the warming of the filament and a delay in brightening the lamp. In many cases this is undesirable or even unacceptable performance. In some transistor dimmers, the practice has been to shut off current very soon after an overcurrent is detected, keep the current off for the remainder of the half-cycle in which the overcurrent occurs, and restart the current in the next half-cycle. If the load is a lamp, the filament warming which occurs in each half-cycle causes higher load resistance, and thus lower peak current to flow in each succeeding half-cycle. This type of current limiting may result in a very low duty cycle in the case of a large cold lamp load, and thus a long time may pass before enough filament warming occurs to allow full current flow without momentary overcurrents.
The previous discussion applies both to reverse phase control and forward phase control. However, in reverse phase control the load current is switched on at or near the zero-cross point, and thus starts very low and increases as the instantaneous line voltage increases. Power is delivered until the necessary cutoff time for the desired output voltage. If a large, cold incandescent load is present, overcurrent may occur an appreciable time after current begins to flow. If the current is shut off at this time, enough power may have been delivered to the lamp filament load to warm it significantly.
In contrast, in forward phase control the load current is often switched on while the instantaneous line voltage is already relatively high. If a large, cold incandescent lamp load is present, overcurrent usually occurs a relatively short time after current begins to flow. The current cutoff at this time results in a low duty cycle and little output power. In this condition, lamp filaments will not warm up quickly and a long time will elapse until the lamp warms enough for normal operation. If the load is near the dimmer's maximum rating, and the current switch-on time is near the line voltage maximum, the filaments may never get warm enough to allow the lamp to light as desired.
Some prior art dimmers have used microprocessors with internal or external A/D convertors, which require repeated readings to determine whether an overcurrent has occurred. This may require prohibitively expensive A/D converters, if continuous overcurrent detection is required. Microprocessors allow sophisticated control functions, but are subject to more stringent power supply requirements than analog circuits and can be relatively expensive to implement. Also, relatively sophisticated and expensive microprocessors have been required to achieve smooth transitions in lamp brightness.